This application is directed to an apparatus and method for conducting pressurized fluid from a source of supply to a delivery point through a metal-to-plastic transition fitting. More particularly, this invention relates to a gas riser which is used to deliver natural gas from a main to a point of use through plastic and metal conduits, tubes or pipes, as well as a method of assembling such a gas riser.
Conventional gas risers comprise a metal casing or pipe at one end and a plastic pipe or tube at the other end. The metal pipe is typically connected to a delivery member such as a gas meter manifold at a location above ground. The plastic tube, on the other hand, is connected to an underground service line which is oftentimes a plastic pipe construction. The metal pipe and plastic tube are joined together at a transition joint. For example in a double walled riser, an outer metal pipe is provided at locations above ground and surrounds the inner plastic tube. The transition between the plastic tube and metal pipe in the gas riser includes a seal, examples of such structures including U.S. Pat. Nos. 3,987,820 and 4,482,170.
Different city and state government codes have their own requirements regarding gas riser constructions. These codes require different wall thicknesses and seal constructions so that a gas riser manufacturer must provide a wide variety of structural arrangements. From an assembly standpoint, this requires a large amount of customized manufacturing in order to meet the different code requirements. Therefore, it has been deemed desirable to provide a gas riser structural arrangement that is easily adapted to various code requirements.
Still other prior art gas riser arrangements provide alternative plastic-to-metal transition joints. Some of these arrangements, though, are deemed to be more difficult to assemble or omit more desired features since the plastic necessitates such an arrangement. For example, it is often desired to provide an epoxy coating on the outer metal casing for corrosion protection. Preferably, the entire length of the outer casing of the gas riser would be coated with the epoxy in a single process step. This would provide an end-to-end epoxy coated outer metal casing that is less subject to corrosion problems, as opposed to other coated casings that apply the epoxy in multiple steps.
Moreover, a preferred method of applying the epoxy coating to the metal casing uses elevated temperatures that could adversely affect any elastomer seal or plastic pipe that may be contained in the casing if necessary precautions or design alterations are not undertaken to protect the elastomer seal or plastic tube. Therefore, it is preferred to complete the assembly of the gas riser outer casing in its entirety, including epoxy coating, from end to end and be able to subsequently install the plastic and elastomeric components.
In a similar vein, the outer metal casing is usually a multipart structure in which the metal components of the casing are joined together. For example, a major portion of the outer casing can be provided by a metal tube or conduit. It is then adapted to receive various size pipe fittings or nipples at an upper, terminal end for connection to a gas meter manifold. A preferred manner of connecting the pipe fitting and the remainder of the metal casing is to weld the two components together. Because of the potential for elevated temperatures associated with the welding operation adversely affecting any plastic or elastomeric component, the metal-to-plastic joint transition is preferably at an area spaced from the weld region. Alternatively, other arrangements can be made to protect the plastic/elastomer members from the high temperature.
If, on the other hand, the outer metal casing is completely assembled end-to-end and epoxy coated, it has heretofore been deemed unworkable to subsequently install the inner plastic tube and assure a proper metal-to-plastic sealed connection. The subject invention is deemed to meet these needs and others in a simple, economical manner.